Single transistor (1-T) dynamic random access memory (DRAM) devices, are a capacitor-less memory that have been recently used in a variety of applications. Also, 1-T DRAM devices may be fabricated by using a relatively simple fabrication process and have an improved sensing margin. 1-T DRAM is also sometimes referred to as “Z-RAM,” which is short for “zero capacitor RAM.”
1-T DRAM offers performance similar to the standard six-transistor SRAM (Static RAM) cell used in cache memory, but since it uses only a single transistor it offers much higher densities. It is also denser than conventional one-transistor, one-capacitor DRAM used for the majority of a modern computer's main memory.
1-T DRAM relies on an effect known as the floating body effect, which was first encountered in CPU design based on the silicon-on-insulator (SOI) process introduced in the early 2000s. This effect causes capacitance to form between the transistor and the underlying insulating substrate, which was seen as a problem that needed to be solved in conventional designs. The same effect, however, allows a DRAM-like cell to be built using the transistor only, the floating body effect taking the place of the conventional capacitor. Consisting of only one part instead of two, 1-T DRAM offers twice the density of DRAM, and five times that of SRAM.
1-T DRAM also uses small cell size leads, which causes 1-T DRAM to be faster than even SRAM, with SRAM normally much faster than DRAM. SRAM's large cell size means that any “reasonable” amount of SRAM cache takes up a large portion of the CPU die. The long traces needed to carry current into the cells have a capacitance of their own, and requires the driver circuitry to “slow down” in order to allow the charge to settle. Although 1-T DRAM's individual cells are not as fast as SRAM, the lack of the long lines allows a similar amount of cache to be run at roughly the same data rates by avoiding this delay while taking up less space.
Despite the utility of 1-T DRAM devices, production has posed unique challenges and methods of operating such semiconductor devices have been limited, particularly with respect to setting an operating mode.